Embedded speaker protection for automotive audio power amplifier

ABSTRACT

A method of operating a speaker system including a speaker coupled to an amplifier, and a dedicated digital speaker protection circuit includes turning on the amplifier in a mute mode, after a first delay period, issuing a play command to the amplifier to place the amplifier in a play mode, but without an input signal during a second delay period, and performing a speaker offset detection during the second delay period, wherein, if there is an offset, then the amplifier is forced back into the mute mode, and if there is no offset, then the amplifier is allowed to continue to operate in the play mode. The method also includes issuing a speaker protection control signal or command if an offset is detected.

FIELD OF THE INVENTION

The present invention relates to audio speaker protection, and, moreparticularly to an embedded speaker protection method and circuitembodiment thereof for an automotive audio power amplifier, which canprotect the speakers automatically in case of high offset during aturn-on procedure.

BACKGROUND OF THE INVENTION

In automotive power amplifier devices an offset detector function isnormally embedded. Any DC output offset exceeding ±2V is signaled out nomatter if there is an input signal or not. This abnormal offset, whenthere is no input signal, might occur as a consequence of initiallydefective or aged and worn-out input capacitors feeding a DC componentto the input. The abnormal offset can put the speakers at risk ofoverheating, or damage the speakers directly.

In current applications of automotive audio power amplifiers, if anaudio system is to protect the speakers in case of high output offset,an MCU (microprocessor) must first read the offset detector outputs fromthe amplifier, and put the amplifier in a mute mode if a high offset isdetected. All these actions require extra MCU operations and arecontrolled outside of the power amplifier IC.

To read the information given by the power amplifier (for example in theTDA7563B quad power amplifier) the diagnosis has to be performed with alow-level output AC signal (or Vin=0). An MCU enables the offsetdetection by setting an I²C command IB1D<5>=1 and then reads the statusof the amplifier after a selectable testing time. An excess offsetgreater than ±2V is signaled out if it is persistent throughout theassigned testing time. The MCU then can decide to turn off or mute thepower amplifier, or just do nothing.

If the MCU ignores all the operations described above, the poweramplifier may maintain play mode with a high offset feeding to thespeakers without any protections, which is undesirable for the reasonsgiven above.

Referring now to FIG. 1, a flow chart 100 shows the turn-on sequence forthe prior art amplifier described above. An MCU enables the offsetdetection, and sets the IBID<5> bit to a logic one at step 102. The MCUthen reads the offset status at step 104. A decision block 106 askswhether or not the offset read is greater than two volts. If no, thenthe audio amplifier maintains its current working state at step 108. Ifyes, then the MCU sends a mute command to the amplifier, or simply turnsthe amplifier off at step 110.

Referring now to FIG. 2, a plot of the voltage at the two speaker outputterminals 202 and 204 is shown in a turn-on procedure in the case of anabnormal offset without MCU intervention according to the prior art. Thestep function in traces 202 and 204 are caused by the abnormal offset inthe speaker system.

What is desired is a more direct method and corresponding circuitapparatus for providing the prior art functionality more quickly, andwithout incurring the extra MCU cycles described above.

SUMMARY OF THE INVENTION

According to the present invention a method of operating a speakersystem including a speaker coupled to an amplifier, and a dedicateddigital speaker protection circuit comprises turning on the amplifier ina mute mode, after a first delay period, issuing a play command to theamplifier to place the amplifier in a play mode, but without an inputsignal during a second delay period, and performing a speaker offsetdetection during the second delay period, wherein, if there is anoffset, then the amplifier is forced back into the mute mode, and ifthere is no offset, then the amplifier is allowed to continue to operatein the play mode. A microprocessor issues a command to place theamplifier in the mute and play modes. The first delay period is about asecond. And the second delay period is about 100 milliseconds. Forcingthe amplifier back into the mute mode is accomplished by the dedicateddigital circuit associated with the amplifier. The offset detectioncomprises a detection of a threshold offset greater than about twovolts. The method of the present invention further comprises issuing aspeaker protection control signal or command if an offset is detected.

A speaker system according to the present invention comprises a speakercoupled to an amplifier, an offset comparator coupled to the speaker forproviding an offset control signal, a digital circuit having a firstinput coupled to an output of the offset comparator, a second input forreceiving an amplifier control signal, a third input for receiving aplay control signal, and an output for providing a forced mute signal,and a microprocessor for controlling the amplifier.

The digital circuit comprises an intermediate node for providing aspeaker protection control signal, a first latch for receiving theoffset control signal, a first logic gate for receiving the play controlsignal, the offset control signal, and the speaker protection controlsignal, a second logic gate coupled to the first latch for receiving theplay control signal and the speaker protection control signal, a secondlatch coupled to the first logic gate for providing the forced mutesignal, and a third latch coupled to the second logic gate and to theintermediate node. The first, second, and third latches are clocked bythe amplifier control signal. The first, second, and third latchescomprise D-type flip flops. The first and second logic gates comprisethree-input NAND gates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a speaker protection procedure according to theprior art;

FIG. 2 is a plot of the voltage at the two speaker output terminals in aturn-on procedure in the case of an abnormal offset without MCUintervention according to the prior art;

FIGS. 3A and 3B taken together is a simplified circuit diagram for adedicated digital speaker protection according to the present invention;

FIG. 4 is a flowchart of a speaker protection procedure according to thepresent invention; and

FIG. 5 is a plot of the voltage at the two speaker output terminals inwhich the amplifier is quickly muted after the detection of a highoffset condition, according to the present invention.

DETAILED DESCRIPTION

According to the present invention, a speaker protection function isembedded inside the power amplifier device itself, by means of adedicated digital circuit.

A simplified circuit diagram for the digital circuit 340 that providesthe speaker protection function is shown in FIGS. 3A and 3B, which issplit into two parts for sake of clarity. The flip-flop enabling signalON goes high when the amplifier is turned on. The FORCEMUTE signal goesto the reference block 330 to control the mute state of the wholeamplifier 320. In the reference block 330, if FORCEMUTE is high, a lowoutput is provided by the play/mute comparator 332.

A speaker system 300 includes a speaker 302 coupled to an amplifier, anoffset comparator 304 coupled to the speaker for providing an offsetcontrol signal at node 305, a digital circuit (306, 308, 310, 312, 314)having a first input coupled to an output of the offset comparator 304,a second input for receiving an amplifier control signal (ON), a thirdinput for receiving a play control signal at node 307, and an output forproviding a forced mute signal (FORCEMUTE), and a microprocessor (MCU)325 for controlling the amplifier.

The digital circuit includes an intermediate node 309 for providing aspeaker protection control signal (SPKPROT), a first latch 306 forreceiving the offset control signal at node 305, a first logic gate 308for receiving the play control signal at node 307, the offset controlsignal at node 305, and the speaker protection control signal SPKPROT, asecond logic gate 310 coupled to the first latch 306 for receiving theplay control signal and the speaker protection control signal, a secondlatch 312 coupled to the first logic gate 308 for providing the forcedmute signal FORCEMUTE, and a third latch 314 coupled to the second logicgate 310 and to the intermediate node 309.

The first, second, and third latches 306, 312, and 314 are D-typeflip-flops clocked by the amplifier control signal (ON), wherein theinverted Q output is coupled to the D-input. CP is the positive edgetriggered clock input and CD is the clear active low input, which clearsthe output when it is low. The first and second logic gates 308, 310 arethree-input NAND gates.

The speaker protection method of the present invention is describedbelow:

The power amplifier generates two internal signals, namely SPKPROT andFORCEMUTE.

The MCU turns on the amplifier in mute mode, and SPKPROT signal islatched high.

After about one second, the MCU sends a PLAY command to the amplifier,and makes sure no signal is applied to the inputs of the amplifier forabout 100 ms.

After the MUTE to PLAY transition time (which is about 40 ms) theamplifier checks the output offset status using internal comparatorsand:

if there is high offset and SPKPROT=1, FORCEMUTE is latched high and theamplifier forces itself back into MUTE mode, and SPKPROT remains at alogic one. A bit of an I²C bus output is set to a logic one to informthe speaker system that the speaker protection is triggered. The MUTEmode is locked and can be reset only when MCU turns off the amplifier bythe I²C bus.

if there is no offset greater than two volts, and SPKPROT=1, FORCEMUTEis latched low which allows amplifier to work in PLAY mode and SPKPROTis reset to a logic zero, which keeps the new speaker protectionfunction inactive in order not to mute the output in case an inputsignal is applied.

The new embedded speaker protection according to the present inventionrequires the MCU to turn on the audio power amplifier in a propersequence. The MCU should first turn on the device in MUTE condition and,after a suitable time for the device to fully work, send a PLAY commandto the device and make sure there is no input signal applied to any ofthe inputs for at least 100 ms.

If the MCU turns on the amplifier directly to play mode without enoughsetting time for the amplifier to be stable, the speaker protectionmechanism of the present invention may not work.

If there is external signal applied to any of the amplifier inputsduring this turn-on phase, the new speaker protection mechanism may givean improper mute instruction to the amplifier itself.

Referring now to FIG. 4, the method 400 of operating a speaker systemincluding a speaker coupled to an amplifier, comprises turning on theamplifier in a mute mode 402, after a first delay period, issuing a playcommand to the amplifier to place the amplifier in a play mode, butwithout an input signal during a second delay period 404, and performinga speaker offset detection 406 during the second delay period, wherein,if there is an offset, then the amplifier is forced back into the mutemode 412, and if there is no offset, then the amplifier is allowed tocontinue to operate in the play mode 410. A microprocessor issues acommand to place the amplifier in the mute mode and the play mode.Forcing the amplifier back into the mute mode is accomplished by thededicated digital circuit in the amplifier, which is shown in FIG. 3.The offset detection comprises a detection of a threshold offset greaterthan about two volts. The method of the present invention furthercomprises issuing a speaker protection control signal or command if anoffset is detected.

The results of the speaker protection method of the present invention isverified in FIG. 5. FIG. 5 shows that the device is set to PLAY aboutone second after turn-on. After the MUTE to PLAY transition the devicedetects a high offset which is above two volts and then mutes itself.Note that speaker output terminals 502 and 504 show a glitch when theoffset is detected, which is quickly suppressed before any damage canoccur to the speaker (or speakers).

Although the invention has been described and illustrated with a certaindegree of particularity, it is understood that the present disclosurehas been made only by way of example, and that numerous changes in thecombination and arrangement of parts can be resorted to by those skilledin the art without departing from the spirit and scope of the invention,as hereinafter claimed.

We claim:
 1. A method of operating a speaker system including a speakercoupled to an amplifier, comprising: turning on the amplifier in a mutemode; after a first delay period, issuing a play command to theamplifier to place the amplifier in a play mode, but without an inputsignal during a second delay period; and performing a speaker offsetdetection during the second delay period; wherein, if there is anoffset, then the amplifier is forced back into the mute mode, and ifthere is no offset, then the amplifier is allowed to continue to operatein the play mode.
 2. The method of claim 1, wherein a microprocessorissues a command to place the amplifier in the mute mode.
 3. The methodof claim 1, wherein a microprocessor issues a command to place theamplifier in the play mode.
 4. The method of claim 1, wherein the firstdelay period is about a second.
 5. The method of claim 1, wherein thesecond delay period is about 100 milliseconds.
 6. The method of claim 1,wherein forcing the amplifier back into the mute mode is accomplished bya circuit in the amplifier.
 7. The method of claim 1, wherein the offsetdetection comprises a detection of a threshold offset greater than abouttwo volts.
 8. The method of claim 1, further comprising issuing aspeaker protection control signal or command if an offset is detected.9. An apparatus for operating a speaker system including a speakercoupled to an amplifier, comprising: a first circuit for turning on theamplifier in a mute mode; a second circuit for after a first delayperiod, issuing a play command to the amplifier to place the amplifierin a play mode, but without an input signal during a second delayperiod; and a third circuit for performing a speaker offset detectionduring the second delay period; wherein, if there is an offset, then theamplifier is forced back into the mute mode, and if there is no offset,then the amplifier is allowed to continue to operate in the play mode.10. The apparatus of claim 9, wherein a microprocessor issues a commandto place the amplifier in the mute mode.
 11. The apparatus of claim 9,wherein a microprocessor issues a command to place the amplifier in theplay mode.
 12. The apparatus of claim 9, wherein the first delay periodis about a second.
 13. The apparatus of claim 9, wherein the seconddelay period is about 100 milliseconds.
 14. The apparatus of claim 9,wherein forcing the amplifier back into the mute mode is accomplished bya circuit in the amplifier.
 15. The apparatus of claim 9 wherein theoffset detection comprises a detection of a threshold offset greaterthan about two volts.
 16. The apparatus of claim 9, further comprising afourth circuit for issuing a speaker protection control signal orcommand if an offset is detected.
 17. An apparatus, comprising: at leastone processor; and at least one memory including computer programinstructions, wherein the at least one memory and computer programinstructions are configured to, with the at least one processor, causethe apparatus at least to: turn on the amplifier in a mute mode; after afirst delay period, issue a play command to the amplifier to place theamplifier in a play mode, but without an input signal during a seconddelay period; and perform a speaker offset detection during the seconddelay period; wherein, if there is an offset, then the amplifier isforced back into the mute mode, and if there is no offset, then theamplifier is allowed to continue to operate in the play mode.
 18. Acomputer program product, comprising at least one non-transitorycomputer readable storage medium having a computer readable program codeportion stored thereon, the computer readable program code portioncomprising: program code instructions for turning on the amplifier in amute mode; program code instructions for after a first delay period,issuing a play command to the amplifier to place the amplifier in a playmode, but without an input signal during a second delay period; andprogram code instructions for performing a speaker offset detectionduring the second delay period; wherein, if there is an offset, then theamplifier is forced back into the mute mode, and if there is no offset,then the amplifier is allowed to continue to operate in the play mode.